This invention relates to mobile radio systems, and more especially it relates to automatic power control systems for use therein.
The term mobile radio system as used herein includes systems having a base station in mutual radio communication with a plurality of portable, transportable or mobile, e.g., vehicularly radio units.
In cellular mobile radio systems it has been shown that it is possible to obtain improved system capacity through the use of automatic control of the transmitted power (herein referred to simply as "power control") of the mobile radios. This reduces the transmitted power to the absolute minimum necessary to obtain acceptable communications. This minimum power for a given mobile radio is a function both of its radio path characteristics to, and the interference at the base station with which it is communicating.
Known power control systems make use of a feedback control link in which the required power is, in some suitable way, signalled back from the base station to the mobile unit to enable the mobile unit to set a correct power level. Such a system will work well provided the number of active users does not exceed the available system capacity at any given time. However, when this capacity is exceeded, there is a mechanism for instability in the overall power control system.
This mechanism arises, as will hereinafter be described, due to the formation of a positive feedback loop involving at least two base stations and at least two mobile radios, one operatively associated with each of the base stations and on the same or similar channel frequency, whereby each base station receives the signal from two or more mobile radios. Thus, in addition to a wanted signal from one mobile radio at least one other signal is received from another mobile radio which is regarded as noise. When this situation arises, two base stations may signal a requirement to their associated mobile radio for more power (to improve the signal to noise ratio), with the rather obvious result that the system will become unstable to the extent that maximum power will eventually be transmitted by both mobile radios which will in turn effectively prevent communication.